1. Field of the Invention
This invention relates to a process for oxidizing monosaccharides using an improved palladium catalyst.
Oxides of monosaccharides, for example, gluconic acid and its salts, are widely used as chelating agents, agents for washing objects made of glass and metals such as iron and aluminum, detergent builders, concrete additives, medicines, food additives and the like. Further, derivatives of these monosaccharide oxides are used in various fields, and especially .delta.-gluconolactone is well known as an effective food additive (for example, a baking powder, a bean curd coagulating agent and a pH adjusting agent).
2. Description of the Prior Art
At present, gluconic acid is prepared mainly by a fermentation process. Glucose can also be oxidized by a chemical oxidation process using bromine or the like, an electrolytic oxidation process and a catalytic oxidation process using a catalyst. Among the current industrial processes, the fermentation process can be performed most easily and simply and is most advantageous from the economic viewpoint. However, this process involves various difficulties. For example, separation of bacterial cells, control of the formation of by-products and disposal of the waste waters produced thereby are troublesome and are not easy to perform.
Preparation of gluconic acid by catalytic oxidation of glucose, as a typical example of oxidation reactions of monosaccharides, is disclosed in, for example, Japanese Patent Publication No. 7620/58. According to this known process, the oxidation is carried out in the presence of a catalyst of a noble metal such as platinum or palladium, under an alkaline condition. In this process, however, even though a 2 wt.% palladium-supported catalyst is used in a large amount such as about 10 wt.% (at least 0.05% by weight of palladium metal, based on the starting glucose), 7 hours are required for completion of the reaction.
In the process for preparing gluconic acid by catalytic oxidation of glucose, an alkaline substance must be added in order to neutralize the formed gluconic acid, and if no alkaline substance is added, the rate of oxidation of the glucose is very low or the oxidation does not progress at all. When glucose is allowed to stand still in an alkaline solution at room temperature, it is readily isomerized to fructose or the like (see Compiled Organic Chemistry, vol. 3, page 167; published by Asakura Shoten in 1957), and in this case, since conversion or oxidation of the thus-formed fructose to glucose is very difficult, the yield of gluconic acid is reduced. In order to prevent the occurrence of this undesirable conversion of glucose to fructose or the like, it is necessary to shorten the time during which glucose is present in an alkaline solution by shortening the time for the oxidation reaction of glucose.